This invention relates to an optical repeater monitoring system and a method thereof, and more specifically, to a system and a method thereof for transmitting monitored information of a repeater and the like to a terminal station in an optical transmission system.
In an optical transmission system, especially in an optical repeatered transmission system comprising at least one optical repeater for optically amplifying and repeating optical signals, it is necessary to remotely monitor and control an operating state and the like of the optical repeater. In a conventional system, for the purpose of transmitting an operating state of an optical repeater to a terminal station, a local oscillating signal source having a individual or common frequency is disposed in each optical repeater and an output of the signal source is modulated with a repeater monitoring information data and transmitted to the terminal station.
The oscillation frequency of the local oscillation signal source, however, fluctuates due to a temperature variation as well as aging and therefore the terminal stations are required to prepare a wider receiving bandwidth in anticipation of the frequency fluctuation. Therefore, in the conventional systems, signal-to-noise ratio (SNR) is deteriorated owing to the inefficiently wider bandwidth.
An object of the present invention is to solve the aforementioned problems and provide an optical repeater monitoring system and a method thereof for receiving a monitor signal at a high SNR.
An optical repeater monitoring system, according to the invention, comprises an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated from an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing a operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. The system further comprises a demodulating signal generator for generating a demodulating signal from either of the output from the oscillating source and the reference signal, the demodulating signal having a frequency equal to that of the monitor signal carrier, a second photodetector for photodetecting the light propagated on the second fiber, and a monitor signal demodulator for demodulating the monitor signal from outputs of the second photodetector and demodulating signal generator.
With the above-mentioned configuration, it is no longer necessary to dispose local oscillator in an optical repeater since a carrier, which transmits a monitor signal showing an operating state of the optical repeater to a terminal station, can be generated in the optical repeater out of a reference signal from the same or another terminal station. As a result, a receiving side of the monitor signal has no need to consider a frequency fluctuation of a carrier for carrying the monitor signal and therefore it is also not necessary to dispose a receiver having an inefficiently wide bandwidth for receiving the monitor signal. Since synchronous detection can be used for demodulating the monitor-signal-modulated signal, the monitor signal can be demodulated at a high SNR. The monitor signal can be received at either of terminal stations; the one transmits the reference signal or another one.
When the reference signal is superimposed on a transmission signal light, an optical fiber transmission line can be effectively utilized. Also, when a dedicated light is used for carrying the monitor signal, a bad influence on the signal light can be reduced.
The optical repeater monitoring method, according to the invention, comprises a reference signal transmitting step for transmitting a reference signal having a predetermined frequency from a reference signal transmitter toward an optical repeater through an optical fiber line, a carrier generating step in the optical repeater for generating a carrier from the reference signal; the carrier has a frequency different from that of the reference signal and carries a monitor signal of the optical repeater, a monitor signal modulating step in the optical repeater for modulating the carrier with the monitor signal, a monitor signal transmitting step in the optical repeater for transmitting the modulated wave of the monitor signal by the monitor signal modulating step to a monitor signal receiver, a demodulating signal generating step in the monitor signal receiver for generating a demodulating signal having a frequency equal to that of the carrier, and a monitor signal demodulating step in the monitor signal receiver for demodulating the monitor-signal-modulated signal from the optical repeater is demodulated with the demodulating signal.
The above-mentioned configuration produces advantages similarly to the optical repeater monitoring system according to the invention.